The word “distributed” in computer science originally referred to computer networks where individual computers were physically distributed within a geographical area. Current usage of the term, however, is much broader and can also refer to autonomous processes that run on the same physical computer. While there is no single definition of a distributed system, a distributed system typically has several autonomous computational entities (computers or nodes) that communicate with each other by message passing. A distributed application is a computer program that runs in a distributed system.
A distributed system may have a common goal, or each computer may have its own user with individual needs. The purpose of the distributed system may be to coordinate the use of shared resources or to provide communication services to the users. The system may include different kinds of computers and network links, and the system may change during the execution of a distributed program. Often, each computer has a limited, incomplete view of the system, knowing only its own function.
Distributed systems are used when the application needs a communication network that connects several computers. Additionally, there are many circumstances in which the use of a single computer would be possible in principle, but the use of a distributed system is beneficial because it is more cost-effective to obtain the desired level of performance by using a number of less expensive computers instead of a single more expensive computer. A distributed system can be more reliable than a monolithic system because the function of the system does not depend on a single computer. Moreover, a distributed system may be easier to expand than a system with only one processor.
Examples of distributed systems and applications of distributed computing include telecommunication networks such as telephone networks and cellular networks, computer networks such as the Internet, wireless sensor networks and routing algorithms; network applications such as the world wide web and peer-to-peer networks, multiplayer online games and virtual reality communities, distributed databases and distributed database management systems, network file systems and distributed information processing systems such as banking systems and airline reservation systems; real-time process control such as aircraft control systems and industrial control systems, parallel computation, scientific computing, including cluster computing and grid computing, various volunteer computing projects, distributed rendering in computer graphics and so on. Distributed systems include server data centers, Internet data centers, web farms and so on. Distributed applications include any applications that run on distributed systems, as, for example, Internet services and websites.